1. Field of the Invention
The present invention relates to apparatus for detecting the position of an optical mark and more particularly to an apparatus so designed that during the alignment of a substrate such as a semiconductor wafer or plate for liquid crystal displays the position of each alignment mark formed on the substrate is detected photoelectrically.
2. Description of the Prior Art
In the registration of position (alignment) for a substrate such as a wafer or plate in the photolithographic step or the like, the systems of photoelectrically detecting the alignment marks formed at given positions on such substrate through a microscope objective lens have been used generally in the past. These photoelectric detection systems are also roughly divided into two types: the so-called light beam scanning type in which the marks are respectively scanned by a spot of laser beam or the like so that the resulting scattered beams or diffracted beams from the marks are received by a photomultiplier, photodiode or the like and the other type in which the magnified images of the uniformly illuminated marks are picked up by a television camera (e.g., a vidicon tube or CCD camera) so that the resulting image signals are utilized. In either of these cases, the resulting photoelectric signals are each subjected to waveform processing to determine the center position of the mark.
As these mark position detecting systems, there are known the conventional techniques disclosed in U.S. Pat. Nos. 4,655,598, 4,402,596 and Japanese Laid-Open Patent No. 62-65327, etc. In these conventional techniques, a monochromatic light is used as the scanning beam or the mark illuminating light and it is based on the following two reasons.
(1) In a projection exposure apparatus (stepper) of the type in which the wafer marks are detected through the projection optical system, an illuminating light or laser beam of a single wavelength is used to avoid the large chromatic aberrations of the projection optical system.
(2) A monochromatic laser beam is used so that it is condensed to a tiny spot for effecting a high-brightness and high-resolution detection.
While the use of a monochromatic illuminating light (or beam) has the effect of ensuring a relatively high S/N ratio, the wafers handled in the exposure apparatus are each usually formed all over the surface thereof with a photoresist layer of about 0.5 to 2 .mu.m thick so that an interference pattern is produced at the photoresist layer thereby causing an erroneous detection during the mark position detection. Then, in recent years proposals have been made toward using illuminating light of multiple wavelength type or wider band type.
In the scanning apparatus of the image pickup type, for example, if a halogen lamp or the like is used as the illumination light source and the wavelength bandwidth of the illuminating light is selected about 300 nm (excluding the sensitizing region for the resist), there is caused practically no interference performance between the reflected light from the resist surface and the reflected light from the wafer surface thereby making possible the detection of a clear image. Thus, in the case of the image pickup type, by simply employing a white light (a wider band) for the illuminating light and making the imaging optical system achromatic, it is possible to obtain a position detecting apparatus which is not subjected to the effect of the resist and extermely high in accuracy.
An example of a conventional projection exposure apparatus equipped with such position detecting apparatus will now be described with reference to FIG. 18. As shown in the Figure, the projection exposure apparatus projects and exposes the pattern area PA on a reticle R firmly held on a holder 11 onto a wafer W (or a glass plate) through a projection lens 10. Then, during this exposure the center of the pattern area PA and the center of the corresponding shot area on the wafer W must be registered (aligned).
For this purpose, the wafer W is loaded on a stage 5 so that the stage 5 is moved two-dimensionally so as to align the reticle R with the wafer W. The movement of the stage 5 is effected by detecting the position on a reference coordinate system of a wafer mark MXn which is formed on the wafer W and then moving the stage 5 in accordance with this position information. As shown in FIG. 19A, this wafer mark MXn is in the form of multiple patterns including a plurality of line patterns arranged side by side. These multiple patterns are formed on a scribe line SCL provided around the shot area Sn on the wafer W.
This wafer mark MXn is detected by the optical system of the position detecting apparatus and the optical system is briefly shown as an off-axis type alignment system in FIG. 18. In the Figure, the illuminating light from a halogen lamp 1 is first transmitted through a bundle of optical fibers 2, passed through a lens system 3, a half mirror 4 and a lens 7 and is then reflected by a prism 9 to irradiate substantially vertically onto the wafer W. The reflected light from the wafer W returns through the same path, is reflected by the half mirror 4 through the prism 9 and the lens 7 and is focused on an index plate 13 by a lens 8. The index plate 13 is formed with index marks 30a and 30b. As shown in FIG. 19A, the index marks 30a and 30b are each composed of two-lines pattern consisting of straight line patterns which are extended in the Y direction and arranged at given intervals in the X direction.
The index plate 13 is arranged substantially conjugate to the wafer W through the lens 7 and the lens system 8. Therefore, an image of the wafer mark MXn on the wafer W is focused on the index plate 13 and the image of the wafer mark MXn and images of the index marks 30a and 30b are focused on an image pickup device 17 such as a CCD camera through relay systems 14 and 15 and a mirror 16. Then, in accordance with the resulting image signal from the image pickup device 17, the positional relation (positional deviations) between the index marks (30a, 30b) on the index plate 13 and the wafer mark MXn is detected by a main control system (CONT) 100. The index marks are used due to the drifting of the image scan starting position of the image pickup device 17. It is to be noted that although not shown, an illumination field stop is arranged within the lens system 3 at a position which is substantially conjugate to the wafer W and this field stop regulates the illumination area on the wafer W.
Here, the state of the portion corresponding to the illumination area and observed by the image pickup image 17 is shown in FIG. 19A. The illumination area on the wafer W is composed of an area SA2 corresponding to the wafer mark MXn and areas SA1 and SA3 respectively corresponding substantially to the index marks 30a and 30b, which are on the index plate 13, in the vicinity of the wafer mark MXn. The illumination area is defined so as to extend over the areas SA1 and SA3 on the ground that the light beams returning from the wafer areas SA1 and SA3 are utilized to transmit and illuminate the index marks 30a and 30b on the index plate 13. U.S. Pat. No. 4,962,318 discloses an image alignment technique employing such indices.
Thus, in order that the light for illuminating the index marks 30a and 30b may not be mixed with any noise components from the other marks and circuit pattern, the areas SA1 and SA3 are ones formed with no circuit pattern and mark and these areas are usually mirror finished. Such areas as the areas SA1 and SA3 are hereinafter referred to as forbidden bands.
Referring now to FIG. 19B, there is shown the video signal from the image pickup device 17 which then corresponds to the wafer alignment mark MXn and the index marks 30a and 30b. In the Figure, the ordinate represents the intensity I of the video signal and the abscissa represents the scanning position P of the stage 5. As shown in FIG. 19B, the video signal from the image pickup device 17 has a signal waveform in which the bottom is attained at each of the positions (picture element positions) corresponding to the positions of the index marks 30a and 30b and the edges of the wafer marks MXn. Also, it is assumed that a wafer alignment mark and index marks are provided in the Y direction and the Y-direction marks are detected by an image pickup device 18.
The above-mentioned conventional technique uses the return light from the wafer surface as the illuminating light for the index marks 30a and 30b on the index plate 13. As a result, if the surface of the wafer W is rough due to flaring, etc., the detection light beams from the index marks 30a and 30b contain noise components due to the roughening of the wafer surface. For instance, if any noise components are mixed with the video signal components generated from the image pickup device 17 in correspondence to the index marks 30a and 30b as shown in FIG. 20, it becomes difficult to discriminate the signals corresponding to the index marks 30a and 30b from the detection signal and a situation is caused in which the position of the wafer mark MXn cannot be detected accurately. In FIG. 20, the ordinate represents the signal intensity I and the abscissa represents the scanning position P of the stage 5.
Here, the wafer mark MXn is composed of the multiple patterns so that the signal corresponding to the wafer mark MXn is subjected to averaging processing to reduce the deterioration in the detection accuracy. However, since the forbidden bands (SA1, SA3) on the wafer are areas which reduce the effective usable area on the wafer W, there is a limitation on the ground that these prohibit zones should preferably be as small as possible. Therefore, each of the index marks cannot be composed of many patterns with the result that there is the danger of the averaging effect being deteriorated and the signal waveform portions corresponding to the index marks 30a and 30b being predominated by the noise components due to the effect of the optical characteristics of the wafer surface. As a result, there is caused a problem that the detection accuracy of the index marks 30a and 30b is deteriorated and hence the position detection accuracy of the wafer mark MXn is deteriorated. Also, since the vicinity of the wafer mark MXn on the wafer must be defined as the forbidden bands corresponding to the index marks, there is the disadvantage that the mark area SA (including the wafer mark MXn and the forbidden bands) on the wafer is inevitably increased (the effective usable area of the wafer is decreased).